The present invention relates to a non-aqueous electrolytic (elecrolyte) solution and a non-aqueous electrolyte cell utilizing the same.
Electric cells utilizing non-aqueous electrolytic solution have been widely used as electric sources of various kinds of consumer electronic appliances because of their high voltage and high energy density as well as their reliability such as storage characteristics. As non-aqueous electrolytic solutions, generally used are those comprising a mixture of solvents of high dielectric constant such as propylene carbonate, .gamma.-butyrolactone and sulfolane and solvents of low viscosity such as dimethoxyethane, tetrahydrofuran and 1,3-dioxolane and an electrolyte such as LiBF.sub.4, LiPF.sub.6, LiClO.sub.4. LiASF.sub.6, LiCF.sub.3 SO.sub.3, LiN(CF.sub.3 SO.sub.2).sub.2, LiAlCl.sub.4 and LiSiF.sub.6.
However, the solvents of such non-aqueous electrolytic solutions generally have a low withstand voltage. When charge/discharge cycles are repeated in secondary cells utilizing an electrolytic solution comprising a solvent with a low withstand voltage, the solvent is decomposed by electrolysis and thereby undesirable situations may be observed; for example, generated gases increase the internal pressure of the cells and the decomposition products are deposited and piled on electrodes. These phenomena reduce charge/discharge efficiency of the cells and reduce energy density of the cells and this ultimately leads a shorter cycle-life of the cells. As attempts to improve the durability of electrolytic solutions, it has been proposed that a carbonate having a high withstand voltage such as diethyl carbonate is used instead of conventional solvents having a low withstand voltage such as esters including .gamma.-butyrolactone and ethyl acetate and ethers including 1,3-dioxolane, tetrahydrofuran and dimethoxyethane to suppress the reduction of energy density of cells after repeated charge/discharge cycles (for example, see Japanese Patent Application Laid-open (KOKAI) No. 2-10666).
Negative electrodes of secondary lithium cells are generally composed of metal lithium or lithium alloys. When charge/discharge cycles are repeated in such cells, lithium ions in electrolytic solutions may be deposited on the negative electrodes in a partial manner to generate very reactive needlelike metals, termed "dendrites". If the dendrites are released from the electrodes, there may be some problematic situations; for example, the cycle-life of the cells is shortened due to the self-consumption of the electrodes and the dendrites penetrate separators, which separate the positive electrode and the negative electrode, to cause short circuit.
Meanwhile, because cells having a high energy density are desired, various researches concerning high voltage cells are being conducted from various points of view. For example, secondary cells termed "rocking chair type" have been developed, which comprises a positive electrode of complexed oxide of lithium and transition metal such as LiCoO.sub.2, LiNiO.sub.2 and LiMn.sub.2 O.sub.4 and an negative electrode of carbon materials. Cells of this type are capable of generating cell voltage of 4 V and they are confirmed to be safe in various experiments such as overcharge, external short circuit, needle piercing, crushing and the like because they do not deposit metal lithium. Therefore, they have become widely used for consumer use. However, when much higher energy density and larger scale are contemplated for these cells, further improvements of safety such as enhanced fire resistance would be desired. Currently used electrolytic solutions do not always have a satisfactorily high inflammation point and they do not have self-extinguishing property. Moreover, even in the lithium secondary cells of "rocking chair type" mentioned above, metal lithium may be deposited under severe conditions such as extreme overcharge due to erroneous usage of the cells. Then it is still required to reduce reactivities of an electrolytic solution and metal lithium.
For these requirements, it has been proposed that phosphoric acid esters, known as self-extinguishable compounds, are added to electrolytic solutions (Japanese Patent Application Laid-open (KOKAI) No. 4-184870). And an electrolytic solution using trimethyl phosphate as solvent thereof is proposed (Japanese Patent Application Laid-open (KOKAI) No.1-102862). However, when more than 15% by volume of these compounds described in the above prior art (Japanese Patent Application Laid-open (KOKAI) No. 4-184870) are added to electrolytic solutions, there have been some cases where problems with regard to cell charge/discharge efficiency, cell energy density and cell life-time (cycle-time) occur, though the solutions have acceptable flame resistance.